The national debt would balloon under tax policies championed by three of the four major Republican candidates for president, according to an independent analysis of tax and spending proposals so far offered by the campaigns.

The lone exception is Texas Rep. Ron Paul, who would pair a big reduction in tax rates with even bigger cuts in government services, slicing about $2 trillion from future borrowing.

According to the report released Thursday by U.S. Budget Watch, a project of the bipartisan Committee for a Responsible Federal Budget, former Pennsylvania senator Rick Santorum and former House speaker Newt Gingrich would do the most damage to the nation’s finances, offering tax and spending policies likely to require trillions of dollars in fresh borrowing.

Both men have proposed to sharply cut taxes but have not identified spending cuts sufficient to make up for the lost cash, the report said. By 2021, the debt would rise by about $4.5 trillion under Santorum’s policies and by about $7 trillion under Gingrich’s plan, pushing the portion of the debt held by outside investors to well over 100 percent of the overall economy, the study said.

The red ink would gush a little more slowly under former Massachusetts governor Mitt Romney, the report said. Until this week, Romney had paired $1.35 trillion in tax cuts with $1.2 trillion in spending reductions, leaving the debt rising on a trajectory that closely tracks current policies.

But that changed Wednesday, when Romney proposed to cut federal income tax rates by 20 percent more for all earners, which would slash U.S. revenue by more than $2 trillion over 10 years.

Romney economic adviser Glenn Hubbard said the lost cash would be recovered by closing tax loopholes and boosting economic activity. But until the campaign offers a more specific plan, Budget Watch analysts said Romney’s entire framework would add about $2.6 trillion to the debt by 2021.

Only Paul emerged as a fiscal conservative in the report. His policies would cut tax revenue by more than $5 trillion over the next decade, the report said, but the loss would be offset by more than $7 trillion in spending cuts, including deep reductions in defense and federal health programs.

The report marks the first independent attempt to gauge the overall impact of policies proposed by the GOP candidates on the nation’s $15.4 trillion debt.

“As we enter the thick of the campaign season, no one can ignore the debt issue,” said Maya MacGuineas, president of the Committee for a Responsible Federal Budget, which supports debt-reduction efforts in Washington. “This report is designed to inform the public on the fiscal policies put forward by each of the Republican candidates and stimulate debate on this crucial topic.”

Paul campaign spokesman Gary Howard welcomed the analysis. “It’s not a surprise to us the report found that Congressman Paul’s plan is the only one that doesn’t raise the debt,” Howard said via e-mail. “The critical importance of dealing with our growing debt has been a hallmark issue of Dr. Paul’s campaign and his career.”

Romney spokesperson Andrea Saul declined to directly address the findings, arguing that Romney is the only candidate to lay out a realistic budget framework “that will jump-start the American economy and bring tax relief to middle-income Americans.”

Aides to Santorum and Gingrich did not respond to requests for comment.

The report does not include an analysis of President Obama’s latest spending blueprint, which seeks to reduce borrowing by $3 trillion by 2021. Budget Watch plans to analyze Obama’s request in future reports.

The report does not seek to offer support to any candidate, and its authors have gone to significant lengths to give the campaigns and their developing policy ideas the benefit of the doubt. The report offers three scenarios for each candidate: A “low-debt scenario” is based on the most generous assumptions about the ability of proposals to save money or generate revenue. A “high-debt scenario” is much more stringent.

The numbers cited above are taken from the report’s “intermediate-debt scenario.” which “gives credit for non-specified changes to certain parts of the budget,” such as promises to reduce some forms of spending by a certain percentage, even if the candidate has yet to nail down ways of generating the cash

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